Preclinical data suggest that folinic acid as well as interferon alpha-2b may enhance the antitumor activity of 5-fluorouracil (5-FU). In a phase I trial, we recently showed that interferon alpha-2b (IFN), folinic acid and 5-FU can be safely administered with a 4-hour infusion of 5-FU. We therefore initiated a phase II trial evaluating the efficacy and safety of these three drugs. Forty-five evaluable patients with advanced metastatic colorectal cancer, documented progressive disease, and previously unexposed to chemotherapy were treated with sequential IFN 5 MU/d subcutaneously and folinic acid 200 mg/m2/d as bolus on days 1 to 7 followed by 5-FU in a 4-hour infusion at a dose of 500 mg/m2/d, resulting in a total dose of 3,500 mg/m2/course. This schedule was repeated on day 21. A total of 204 courses of therapy were completed. One of 45 patients (2%) achieved a complete response, and 13 of 45 patients (29%) achieved a partial response. An additional 16 patients (36%) had stable disease. The median time to disease progression was seven months (2 to 24 months). Despite the relatively high-dose intensity of 5-FU, toxicity was very mild. Grade 3 or 4 myelosuppression, stomatitis, and nausea/vomiting occurred in only three of 45 patients (7%). Four of 45 patients (9%) suffered from severe (grade 3/4) diarrhea. Neurotoxicity and infections of grade 2 to 4 did not occur. From these data we conclude that modulation of 5-FU with both folinic acid and IFN induces an overall response rate of 31% in disseminated colorectal cancer. Using a 4-hour application schedule of 5-FU, the therapeutic index can be improved even for high-dose intensity and requires further evaluation in combination with other modulators.